Opaque patterns on translucent backgrounds are often sensed or detected for the purpose of reading, recording, copying, inspecting or viewing the pattern. This is done, for example, when documents are electronically read by an optical reading or copying device and when individual ceramic microcircuit packaging sheets are inspected prior to assembly into multilayer structures.
It is known that a pattern may be sensed or detected in theory without using any imaging optics by positioning a suitable optical pattern sensing apparatus (such as a TV camera tube, a stationary or scanned linear or matrix light detector array, a scanned individual light detector, an optically sensitive film or layer, an image converter, etc.) sufficiently close to the pattern. This is sometimes called proximity pattern detection.
Proximity pattern detection is not usually practical for use with front illumination, however, because too great a distance is required between the detector and the pattern in order to let in the front illumination. As a result, proximity pattern detection is more commonly done by forming an opaque pattern on a non-opaque background (or vice versa) and illuminating the pattern from the back (rear illumination) while positioning a suitable optical pattern sensing apparatus adjacent the front side of the pattern.
Proximity pattern detection using rear illumination works very well when the non-opaque background is transparent, because the use of collimated rear illumination then allows the optical pattern sensing apparatus to be placed a reasonable distance away from the pattern without significant loss of pattern detail or resolution.
Unfortunately, when the opaque pattern to be detected has a tranlucent background, the light scattering which occurs as the light passes through the translucent background requires that the optical pattern sensing apparatus be positioned extremely close to the pattern. This is generally not practical to do, so that projection pattern detection usually is used instead by inserting imaging optics between the opaque pattern and the optical pattern sensing apparatus. The imaging optics projects the light pattern formed by the opaque and translucent areas onto the optical pattern sensing apparatus. Because of the presence of the imaging optics, projection pattern detection is inherently much more complicated and expensive than proximity pattern detection.
The object of this invention is to provide a practical proximity pattern detection technique and apparatus wherein an opaque pattern on a translucent background may be sensed or detected using rear illumination without requiring that the optical pattern sensing apparatus be positioned unreasonably close to the pattern and without requiring any use of imaging optics between the opaque pattern and the optical pattern sensing apparatus.